Fail-safe system and technique



Sept. 17, 1957 w. G. ROWELI.

FAIL-SAFE SYSTEM AND TECHNIQUE 2 Sheets-Sheet l Filed May 8, 1956 Isl ldxlllxl ...IIV d.

Sept. 17, 1957 w. G. ROWELL.

EME-SAFE SYSTEM AND TECHNIQUE 2 Sheets-Sheet 2 Filed May 8, 1956 CIZ IOB

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'IMPULSE DIVIDING PPARATUS I I I l IVENTOR. WILLIAM @.RowELL FIG.2

ATTORNEYS United States Patent O FAIL-SAFE SYSTEM AND TECHNIQUE 'William G. Rowell, Quincy, Mass., assigner to Scully Signal Company, Melrose, Mass., a corporation of Massachusetts Application May S, 1956, Serial No. 583,571

2.3 Claims. (Cl. 340--213) The present invention relates to fail-safe systems and techniques and, more particularly, to systems for and techniques for monitoring events that are accompanied by a characteristic fluctuating `or perturbating condition or phenomenon.

As is disclosed in my copending applications, Serial Nos. 375,224, now Patent No. 2,798,213, and 425,296, now Patent No. 2,798,214, filed on or about August 19, 1953, and April 23, 1954, respectively, and in an article entitled Fail-safe gets new meaning by William G. Rowell and A. B. Van Rennes, appearing on pages 79 through 8l, Control Engineering, March 1956, and in a further article by William G. Rowell, entitled Fail-safe monitor, appearing on pages 28 through 31 of Electronic Design, March l, 1956, monitoring lor other systems of a wide variety of electrical, electronic, mechanical or electro-mechanical types, may now be rendered completely fail-safe. The term fail-safe is intended to connote that the system is incapable of producing a false indication of the presence of an event-to-be-detected or producing an indication that the system is operative when, in fact, one or more components of the system has or have failed. The importance of such a safeguard for the protection of life and property, or, at the very least, for providing economical operation, is stressed in the said applications and articles.

While the systems and techniques discussed in the said applications and articles are entirely satisfactory and have been found to work admirably well in practice, they may be somewhat simplified in the case of the monitoring of events that of themselves are inherently accompanied by characteristic lluctuations. Included in events of this character are flames such as, for example, those produced by oil or gas burners and the like. Flames may be monitored by sensing the visible-light condition accompanying some llames, the infra-red or heat condition accompanying some flames, or the sounds of combustion accompanying the production of a flame. As discussed in the said applications, flames are known to have inherently associated with them fluctuating light, heat and sound characteristics of definite predetermined frequency bands. The range of the rates of fluctuation of the infrared or heat characteristics generated by gas flames and the like, for example, is from about to about 25 cycles per second. The range of sound-frequency fluctuations for the flames of oil burners and the like, as another illustration, includes frequencies of the order of 100 cycles and higher.

Underlying the technique disclosed in application Serial No. 375,224, as explained, also, in the said articles, is the accompanying of a principal signal by a checking or modifying signal of predetermined characteristics. In accordance with the present invention, the event-to-be-detected, provides its own checking-signal or modifyingsignal fluctuation modification of the principal signal, permitting the employment of less apparatus. When so simplified by means of the present invention, however, the systems and methods still operate in accordance with ICC the principles and modes of operation disclosed in the said applications, but they do so with the aid of less equipment.

An object of the present invention, accordingly, is to provide a new, improved and simplified fail-safe system and technique of the character described. In summary, the present invention contemplates the employment of means responsive to a characteristic of an event-to-bedetected, which event inherently has fluctuations of a rate or rates lying within a predetermined range of rates the longest period of which is less than a predetermined time interval. The responsive means produces a signal indication of the presence of such a characteristic modified with uctuations corresponding to the said uctuations of the event. The fluctuation-modilied signal is applied to the input of the monitoring system and is transmitted through the system to the output thereof. Means is provided for recovering or reproducing the fluctuations from the fluctuation-modified signal in the output of the system, as is means for indicating the failure so to recover or reproduce fthe fluctuations for a time substantially equal to the said predetermined time interval. Preferred constructional details are hereinafter presented.

A further object is to provide such a system and technique that are particularly adapted for the monitoring of flames and the like.

Other and further objects will be explained hereinafter and will 'be more particularly pointed out in the appended claims.

The invention will now be described in connection with the accompanying drawings Fig. l of which is a circuit diagram of a preferred embodiment of the invention; and

Fig. 2 is a similar diagram of a modiication.

While, for purposes of illustration, the invention will hereinafter be described in connection with an electronic flame-monitor system, it is to be understood that the invention is yby no means limited thereto and that it may equally well be applied to many diferent kinds of electrical, electronic, mechanical and electro-mechanical systems, as discussed in the said applications and articles.

In Fig. 1, a radiation monitor sensor or detector device is shown at 2, responsive to radiations from a ame 1, such as, for example, a gas or oil burner llame and the like. While the sensor 2 could be a microphone for responding to specific-frequency sound fluctuations characteristically accompanying the production of the llame 1, as before explained, or a photocell for receiving the visible light characteristic, ultra-violet or other conditions of the flame 1, it is preferred to employ a photocell 2 of the lead-sulphide variety and the like, which will respond to the infra-redor heat-fluctuation characteristics of the llame 1, occurring within the said 5 to 25 cycle-per-second range. The photocell infra-red or heat sensor or detector 2 is connected to the input terminals 4, 6 of a high inputimpedance electronic flame-monitor amplifier system 8. The system 8 is shown as a conventional commercial type embodying a pair of alternating-current amplifier stages comprising electron or vacuum tubes TlA and TIB and a pair of further amplifier tubes TSA and T3B of the direct-current type. In the output of the system 8, represented at the terminals 10, 12, shown disposed above the amplifier TSB, a transformer TR2 is provided, having a primary winding 14 electromagnetically coupled to a step-up secondary winding 16. The secondary winding 16 is, in turn, shunted betwen its upper and lower terminals by similarly polarized rectiiiers 18 and 20. An ultimate direct-current load relay RY is connected by a conductor 22 to the junction 24 of the rectiiiers 18 and 20, and by a conductor 26 to an intermediate tap 28 of the secondary winding 16.

The transformer TR2 is tuned, as by appropriate wellknown design of the mass of the magnetic core material associated therewith, to provide a low-impedance substantially maximum response to electrical fluctuations fed thereto of rate or rates within the said to 25 cycle-persecond range or any band within that range. The 'transformer TR2 will then present a high-impedance low-level or negligible response to all other fiuctuation rates of frequencies lying outside the said range such as, for example, the sixty-cycle frequency available at the alternatingcurrent mains terminals 32, 34.

The load relay RY, as before stated, is illustrated as a direct-current type of relay that may control, through an armature 36, a switch 38 that cooperates with either of a pair of contacts 40, 42 in order to operate an alarm, indicator, control or other device, as explained in the said applications and articles. The relay RY is provided with a holding capacitor C10, shunting the same, to provide slow-release-response characteristics. By adjusting the value of the capacitor C and the impedance of the relay winding, the relay RY, after being energized, will deenergize only after the elapse of an interval of time corresponding to the before-mentioned predetermined time y,interval greater than the longest period in the said 5 to cycle-per-second range. Other types of delayed or slow-response indicators, alarms or controls may, of course, also be employed, as discussed in the said applications and articles.

Unlike prior-art monitor circuits of the type illustrated in Fig. l, the fluctuation-modified electrical signal fed into the input terminals 4, 6 of the system 8 and amplified in the pair of alternating-current amplifiers TIA, TIB is not integrated, smoothed-out or rectified to remove the fluctuations before application to the output terminals 10, 12. To the contrary, the fluctuation-modified signal is passed through the further pair of amplifiers T3A, T3B and to the output terminals I0, 12, whence the fluctuations are stepped-up or increased in amplitude by the transformer TR2, and then rectified to hold the relay RY energiaed. So long as the repetitive uctuations within the s a1d range `of rates that were passed from the input termlnals 4, 6 through the system 8 to the output terminals 10, 12 are recovered or reproduced in the transform- -er TR2 from the amplified fluctuation-modified electrical signal, accordingly, the load relay RY will remain energized. Upon failure so to recover or reproduce the fiuctuations for a time substantially equal to the said predetermined time interval, the relay RY will become deenergized and operate the alarm, indicator, control or other apparatus. The complete monitor from the sensor 2 to the rectifiers 18, 20, and including all of the electrical -and electronic components of the system 8, is thus comnected by a plate load resistor R6 and a current-limiting resistor R9 to a plate voltage rectifier TZB. The resistor R9 actually connects to the cathode 54 of the rectifier TZB, the anode or plate 56 of which is connected to an intermediate tap 58 of the secondary winding 60 of a power transformer TR1. The primary winding 62 of the power transformer TR1 is energized from the mains terminals 32, 34. The upper end of the secondary winding 60 is connected to the output terminal 12 and, through the primary winding 14 of the step-up transformer TR2, to the output terminal 10. The lower end of the secondary winding 60 is connected by conductor 64 to the ground terminal 50. Heater windings 66 for the amplifier tubes TIA, TIB, T2A, T2B and T3A and TSB are energized by the further secondary winding 60' of the l power transformer TRI, also provided by the conductor 64.

Associated with the first alternating-current amplifier stage is a type filter network tuned or adjusted to pass the desired range or band of fluctuation frequencies, such as the before-mentioned 5 to 25 cycle-per-second range or any desired band within that range. The filter network is shown comprising a shunt-connected resistor R2 connected from a point 66 between the condenser or capacitor C1 and the resistor R3 to the ground terminal 50, the resistor R3, and a further capacitor C3 connected from the control grid 46 of the tube TIA through a resistor R5 to the ground terminal 50. A direct-current blocking capacitor C4 is connected from the plate or anode SZ of the tube TIA to the junction 68 between the capacitor C3 and the resistor RS. A plate-voltage supply filter-smoothing capacitor C2 is connected between the ground terminal Si) and the left-hand terminal of a current-limiting resistor R4. The right-hand terminal of the resistor R4 connects with the before-mentioned limiting resistor R9. A similar smoothing capacitor C9 is connected from the cathode 54 of the plate-supply voltage rectifier TZB to ground. A voltage-regulator gaseous-discharge diode T4 is disposed between the said right-hand terminal of the current-limiting resistor R4 and the ground terminal Si). Still an additional current-limiting resistor RI is connected between the input terminal 4 of the system 8 and the upper terminal of the capacitor C2.

The output of the normally conducting alternatingcurrent amplifier stage TIA is coupled by the capacitor C5 and resistor R7 to the control-grid electrode 7G of the next successive amplifier stage TIB. The cathode 72 of thetube TIB is connected to the ground terminal 50 and its plate or anode 74 is connected through a plate-load resistor RII to the cathode S4 of the plate-supply voltage rectifier T2B. The amplifier stages TIA and TIB may be disposed in a single envelope, such as the type 12SL7GT double triode, or they may comprise separate tubes. Similar remarks apply in connection with the arnplifier stages T3A, TSB, which may be of the type l2SN7GT, and the diode TZB and a further diode T2A, which, in turn, may be a type 12H6 4double diode. A 1r-type filter network R8, C6, R10, corresponding to the network R2, R3, C3, R5, previously discussed in connection with the amplifier stage TIA, is connected in the input circuit of the second amplifier stage TIB between the control-grid electrode 70 thereof and the cathode 72. A direct-current blocking capacitor C7 is connected between the plate or anode 74 of the tube TIB and the upper terminal of the filter capacitor C6.

The output of the amplifier TIB is coupled by capacitor C8 and resistor R13 to the control-grid electrode 76 of the third amplifier stage T3A of the direct-current type. The resistor R13 serves with the resister R14, the latter of which is connected between the control-grid electrode 76 and the grounded cathode 78 of the tube T3A, as grid-bias resistors. They are shunted by a further resistor R12 that serves with the capacitor C8 as a filter network and serves, also, as the plate-load resistor for the previously mentioned further diode T2A. That diode T2A, in turn, is connected between the coupling capacitor C8 and the ground terminal 50 and furnishes direct-current grid-bias voltage for the amplifier T3A. The amplifier T3A, like the amplifiers TIA and A1B is normally conducting and its plate or anode 80 is connected through a plate-load resistor R15' to a further intermediate terminal 8?. of the secondary winding 60 of the power transformer TR1. The resistor R15 also serves as a grid-bias resistor for the last amplifier stage T3B in View of its connection between the cathode 84 and the control-grid electrode 86 thereof. The plate or anode 88 of the tube T313 is, in turn, connected to the output terminal 10.

l The fluctuation-modified electrical signal applied to the input 4, 6 of the system 8 is thus fed to the normally conducting amplifier TlA, producing amplied fluctuations at the plate 52 that are, in turn, amplified by the second amplifier T1B. The corresponding negativelygoing fluctuating voltages appearing at the capacitor C8 repetitively cut-off or reduce the conduction of the arnplifier TSA. Since the grid 86 of the tube TSB is normally maintained at a negative potential, the cut-off of the tube TSA will reduce this negative potential so that the tube TSB will conduct. Repetitive conduction of the amplifier TSB will result in the reproducing or recovering of the fluctuations in the transformer TR2.

As is explained in the said applications and articles, other types of output devices besides the transformer TR2 may be employed. In Fig. 2, which may be preferred in view of its simplicity and readily obtainable conventional components, accordingly, the terminals 10 and 12 of Fig. 1 are shown connected to an electromagnetic relay RY1 shunted by a holding capacitor represented by the same nomenclature C10 as the holding capacitor of the relay RY and adapted to respond to fluctuations within the said range or band. The relay RY1, moreover, should have sufficiently high alternating-current impedance to prevent its operation at the power or mains frequency, as discussed in connection with the transformer TR2 of Fig. 1. A suppression circuit for maintaining the ultimate load relay or other device RY energized so long as the fluctuations are recovered and reproduced by synchronous energization and deenergization of the relay RY1, is shown as comprising a capacitor C12 for alternately storing direct-current energy and feeding the same to the slow-release load RY in synchronism with the recovery of the fluctuations in the output of the stystem 8. This result is effected by the switching members 90 and 92 that are pivotable between alternate positions in response to the upward movement of the armature 94 of the relay RY1 upon energization of the same, and the downward movement iu response to the restoring power of the spring 96 upon de-energization of the relay RY1. With the relay RY1 deenergized, the switch 90, which is-permanently connected by the conductor 26 to the lower terminals of the load relay RY and its shunt capacitor C10, engages the contact 98 and establishes an electrical-circuit connection by a conductor 100 to a resistor R16. The resistor R16 is connected to the left-hand terminal of the storing capacitor C12. The right-hand terminal of the storing capacitor C12 is connected by the conductor 102 to a contact 104 and thence through the switch member 92 to the conductor 22 that feeds the upper terminals of the load relay RY. In the illustrated position, therefore, the capacitor C12 feeds out its stored energy to the load relay RY. It should be noted, moreover, that the hereinafter described rectifier S is at this time shunted by the low-impedance path of the switching member 92. When the relay RY1 is in its energized condition, however, the switch members 90 and 92 are moved to their upper positions, breaking the connection with contact 98 and making connection with contact 106, respectively. In such position, the capacitor C12 charges or stores directcurrent potential in an electric circuit traceable from the terminal L1 of a source of alternating-current energy A. C., which may be the same source as at the terminals S2, S4 of Fig. l, by way of the conductor 100, the charging impedance R16, the capacitor C12, the rectifier S, the switch member 92, the contact 106, a further conductor 108 and the other terminal L2 of the source. The continual recovery of the desired range of fluctuations in the relay RY1 will therefore maintain the switching means repetitively alternating between its upper and lower positions, thus keeping the load RY energized. Upon failure to recover the fluctuations for a period greater than the before-mentioned predetermined time interval, the relay RY will thereupon provide an appropriate indication.

In all cases, however, ally the numerous unsafe failure possibilities that exist in conventional prior-art monitor circuits 8 of the type illustrated in Fig. -l are eliminated by the present invention. Included in such possibilities are the following conditions which have been observed in practice: an opening in the winding or in a connection in the lower portion of the winding 60 of the transformer TR1; an open-circuit or defective power filter capacitor C2 or C9; a short-circuited plate-supply rectifier T2B; a short-circuit in the grid-to-cathode circuit of the tube TSB or the wiring thereto; or an anode-to-cathode short in the tube TSB, to mention but a few.

If desired, the fluctuation impulses may be counted down or divided so that the relay RY1 of Fig. 2, or the load RY of Figs. 1 and 2, receives energization once for every group of so-many fluctuations. In such a case, the period of the count-down will be less than the previously discussed predetermined time interval. For purposes of illustration, this result is indicated as effected in` Fig. 2 by the dotted block labelled Impulse-Dividing Apparatus, which may assume the form of any well-known cut-down apparatus, such as a multivibrator circuit or a condenser-step charger, and the like. Instead of having irregular fluctuation rates, moreover, the fluctuations may be fed to multivibrator and similar-type circuits for regularizing the fluctuations into a constant rate, as is well known.

In summary, therefore, the perturbations caused by the event to be detected are, in addition to meeting the amplifying and other conditions of the system, also expressly uitilized as a checking signal for determining the operability of the sensor 2, the amplifying system 8, and the output thereof to the coil of relay RY.

As before explained, moreover, other types of sensors may be employed as may other types of monitor, detecting or receiving systems. Resistance, reactance or bridgetype sensors or detectors, as further illustrations, may be employed, as may oscillator circuits and the like. Other types of suppression circuits or impulse-responsive devices, including those disclosedv in the said applications and articles, may also be utilized.

Further modifications will occur to those skilled in the art and all such are considered to fall within the spirit and scope of the invention as defined in the appended claims.

What is claimed is:

l. A system for monitoring an event having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, means responsive to a characteristic of the event for producing an electrical signal indicative of the presence of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the event, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, means for recovering the fluctuations from the fluctuation-modified signal in the output of the system, and means operable in the event of failure so to recover the fluctuations for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

2. A system for monitoring an event having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than predetermined time interval, the system having, in combination, an input, an output, means responsive to a characteristic of the event for producing an electrical signal indicative of the presence of such characteristie and modified with electrical fluctuations corresponding to the said fluctuations of the event, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, means selectively responsive to electrical 3. A system for monitoring an event having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, thc system having, in combination, an input, an output, means responsive to a characteristic of the event for producing an electrical signal indicative of the presence of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the event, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, means comprising a relay of low impedance to electrical fluctuations of rate lying Within the said predetermined range of rates and of high impedance to fluctuations of rate outside the said predetermined range of rates for recovering the fluctuations from the fluctuation-modified signal in the output of the system, and means operable in the event of failure so to recover the fluctuations for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

4. A system for monitoring an event having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, means responsive to a characteristic of the event for producing an electrical signal indicative of the presence of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the event, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, means comprising a stepup transformer selectively responsive to electrical fluctuations of rate lying within the said predetermined range of rates for recovering the fluctuations from the fluctuation-modified signal in the output of the system, and means operable in the event of failure so to recover the fluctuations for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

5. A system for monitoring an event having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, means responsive to a characteristic of the event for producing an electrical signal indicative of the presence of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the event, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, switching means responsive to the recovering of the fluctuations from the fluctuation-modified signal in the output and adapted to occupy alternate positions at the said fiuctuation rate or rates, means for supplying alternating-current potential of a rate outside the said predetermined range of rates, rectifying means, directcurrent potential-storing means, an electric circuit operative when the switching means occupies one of its positions for connecting the supplying means to the storing means through the rectifying means in order to store direct-current potential in the storing means, a slow-response direct-current load adapted to respond in a period of time substantially equal to the said predetermined time interval, and a further electric circuit operative when the switching means occupies its alternate position to connect the storing means to the load. Y

6. A system for monitoring an event having inherent sendos fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, means responsive to a characteristic of the event for producing an electrical signal indicative of the presence of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the event, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, relay-controlled switching means responsive to the recovering of the fluctuations from the fluctuation-modified signal in the output and adapted to occupy alternate positions atthe said fluctuation rate or rates, means for supplying alternating-current potential of a rate outside the said predetermined range of rates, rectifying means, capacitor means, an electric circuit operative when the switching means occupies one of its positions for connecting the supplying means to the capacitor means through the rectifying means in order to store direct-current potential in the capacitor means, a slowresponse direct-current capacitor-shunted load relay adapted, after being energized, to release in a period of time substantially equal to the said predetermined time interval, and a further electric circuit operative when the switching means occupies its alternate position to connect the first-named capacitor to the load relay in order to maintain the same energized.

7. A system for monitoring an event having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, means responsive to a characteristic of the event for producing an electrical signal indicative of the presence of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the event, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, means for recovering the fluctuations from the fluctuation-modified signal in the output of the system and for producing further electrical impulses, one corresponding to a group of the said fluctuations and of period less than the said predetermined time interval, and means operable in the event of failure to receive the said impulses for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

8. A system for monitoring an event having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, means responsive to a characteristic of the event for producing an electrical signal indicative of the presence of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the event, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, means for recovering the fluctuations from the fluctuation-modified signal in the output of the system, a slow-response load adapted, after being energized, to de-energize in a period of time equal to the said predetermined time interval, and means for alternately storing energy and delivering the stored energy to the load synchronously with the recovering of the said fluctuations.

9. A flame-monitor system for monitoring a flame having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, a detector responsive to a characteristic of the flame for producing an electrical signal indicative of the detection of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the llame, means for applying the fluctuation-modified signal to the input of the System to transmit the same through the system to the output thereof, means for recovering the fluctuations from the iluctuation-modiiied signal in the output of the system, and means operable in the event of failure so to recover the fluctuations for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

l0. A llame-monitor system for monitoring a llame having inherent fluctuations of a rate or rates lying within a predetermined range o-f rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, a detector responsive to the infrared or heat characteristic or" the llame for producing an electrical signal indicative of the detection of such characteristic and modified with electrical fluctuations corresponding to the said lluctuations of the llame, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, means for recovering the fluctuations from the lluctuaion-modified signal in the output of the system, and means operable in the event of failure soto recover the iluctuations for a time substantially equal to the said predetermined time interval, lfor thereupon. indicating such failtu'e.

ll. A llame-monitor system for monitoring a llame having inherent lluctuations of a ratte or rates lying within a predetermined range of rates the longest corresponding periold of which is less than a predetermined time interval, the system having, in combination, an input, an output, a detector responsive toA the infra-red or heat characteristic of the llame for producing an electrical signal indicative of the detection of suchcharacteristic and modified with electrical uctuations corresponding to the said fluctuations of the flame, means for applying the iluctuation-modied signal to the input of the system to transmit the same through the system to the output thereof, means selectively responsive to electrical fluctuations of rate lying within the said predetermined range of rates for recovering the fluctuations from the lluctuationmodied signal in the output of the system, and means operable in the event of failure so to recover the lluctu. ations for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

12. A llame-monitor system for monitoring a ila-me having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an, output, a Idetector responsive to the infra-red or heat characteristic of the flame for producing an electrical signal indicative of the detection of such characteristic and modiiied with electrical fluctuations corresponding to the said fluctuations of the llame, means for applying the uctuation-modied signal to the input of the system to transmit the same through the system to the `output thereof, means comprising a relay of low impedance to electrical iluctuations of rate lying within the said predetermined range of rates and of high impedance to lluctuations of rate outside the said predetermined range of rates for recovering the fluctuations from the fluctuation-modified signal in the output of the system, and means operable in the event of failure so to recover the iluctuations for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

13. A llame-monitor system for monitoring a ame having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, a detector responsive to the infra-red or heat characteristic of the llame for producing an electrical signal indicative of the detection of such characteristic and modified with electrical fluctuations corresponding to the said iluctuations of the llame, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the ouput thereof, means comprising a step-up transformer selectively responsive to electrical fluctuations of rate lying within the said predetermined ran-ge of rates for recovering the tluctuations from the lluctuation-modifed signal in the output of the system, and means operable in the event of failure so to recover the tluctuations for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

i4. A flame-monitor system for monitoring a llame having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, aninput, au output, a detector responsive to the infra-red or heat characteristic of the llame for producing an electrical signal indicative of the detection of such characteristic and modified with electrical tluctuations corresponding to the said lluctuations of the llame, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to he ouput thereof, switching means responsive to the recovering of the fluctuations from the fluctuation-modified signal in the output and adapted to occupy alternate positions at the said lucuation rate or rates, means for supplying alternating-current potential of a rate outside the said predetermined range of rates, rectifying means, directcurrent potential-storing means, an electnic circuit operative when the switching means occupies one of its positions for connecting the supplying means to the storing means through the rectifying means in order to store direct-current potential rin the storing means, a slow-respense direct-current load adapted to respond in a period of time substantially equal to the said predetermined time interval, and a further electric circuit operative when the switching means occupies its alternate position to connect the storing means to the load.

l5. A llame-monitor system for monitoring a flame having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, a detector responsive to the infrared or heat characteristic of the llame for producing an electrical signal indicative f' trolled switching means responsive to the recovering of the iluctuations from 4the fluctuation-modified signal in the output and adapted to occupy alternate positions at the said uctuation rate -or rates, means for supplying alternating-current potential of a rate outside the said predetermined range of rates, rectifying means, capacitor means, an electric circuit operative when the switching means occupies one of its positions for connecting thc supply means to the capacitor means through the rectifying means in order to store direct-current potential in the capacitor means, a slow-response direct-current capacitorshunted load relay adapted, after being energized, to release in a period of time substantially equal to the said predetermined time interval, and a further electric circuit operative when the switching means occupies its alternate position to connect the first-named capacitor to the load relay in order to maintain the same energized.

16. A flame-monitor system for monitoring a llame having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, a

detector responsive to the infrared or heat characteristic of the flame for producing an electrical signal indicative of the detection of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the flame, means for applying the fluctuationmodified signal to the input of the system to transmit the same through the system to the output thereof, means for recovering the fluctuations from the fluctuation-modified signal in the output of the system and for producing further electrical impulses, one corresponding to a group of the said fluctuations and of period less than the said prcdetermined time interval, and means operable in the event of failure to receive the said impulses for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

17. A flame-monitor system for monitoring a flame having inherent fluctuations of a rate or rates lying within a predetermined range :of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, a detector responsive to the infrared or heat characteristic of the flame for producing an electrical signal indicative of the detection of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the flame, means for applying the fluctuationmodified signal to the input of the system to transmit the same through the system to the output thereof, means for recovering the fluctuations from the fluctuation-modi fied signal in the output of the system, a slow-response load adapted, after being energized, to de-energize in a period of time equal to the said predetermined time interval, and means for alternately storing energy and delivering the stored engery to the load synchronously with the recovering of the said fluctuations.

18. A flame-monitor system for monitoring a flame having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, amplifier means connected between the input and the output and adapted selectively to respond to electrical fluctuations lying Within the said predetermined range of rates, a detector responsive to the infra-red or heat characteristic of the flame for producing an eelctrical signal indicative of the detection of 'such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the flame, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the amplifier means to the output of the system, means for recovering the fluctuations from the fluctuation-modified signal in the output of the system, and means operable in the event of failure so to recover the fluctuations for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

19. A flame-monitor system for Imonitoring a flame having inherent fluctuations of a rate or rates lying within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, a pair of successively connected alternating-current amplifier stages and a pair of successively connected amplifier stages of the direct-current type connected between the input and the output of the system, the first stage of each of the pair of stages being normally conductive and the alternating-current amplifier stages having means selectively responsive to fluctuations within the said predetermined range of rates, a detector responsive to the infra-red or heat characteristic of the flame for producing an electric signal indicative of the detection of such characteristic and modified with electrical fluctuations corresponding to the said fluctuations of the flame, means for applying the fluctuation-modified signal tothe input of the system to transmit the same through the pairs of amplifier stages to the output of the system, means for recovering the fluctuations from the fluctuation-modified signal in the output of the system, a slow-response loa'd adapted, after being energized, to de-energize in a period of time equal to the said predetermined time interval, and means for alternately storing energy .and delivering the stored energy t-o the load synchronously with the recovering of the said fluctuations.

20. A system for monitoring an event having inherent fluctuations of a rate or rates lying Within a predetermined range of rates the longest corresponding period of which is less than a predetermined time interval, the system having, in combination, an input, an output, means responsive to a characteristic of the event for producing a signal indicative of the presence yof such characteristic and modified with fluctuations corresponding to the said fluctuations of the event, means for applying the fluctuation-modified signal to the input of the system to transmit the same through the system to the output thereof, means for recovering the fluctuations from the fluctuation-modified signal in the output of the system, and means operable in the event of failure so so recover the fluctuations for a time substantially equal to the said predetermined time interval, for thereupon indicating such failure.

21. A signal-detection system adapted to prevent a false indication from being given when a characteristically fluctuating event-to-be-detected is not present or when any system component has failed, the system having, in combination, a sensor device adapted to detect the fluctuating condition characteristically produced by the event-to-bedetected and arranged, when influenced by the said condition, to produce a corresponding fluctuating electrical signal, an amplifier connected to the sensor to amplify the said fluctuating electrical signal, electromagnetic means associated with the said amplifier and adapted to be recurrently energized by the amplified fluctuating electrical signal, the electromagnetic means providing maximum response to fluctuating electrical signals of a band of frequencies corresponding to the frequencies of fluctuation of the event, and indicating means responsive to the said electromagnetic means and adapted t-o give a steady indication during the time that the said electromagnetic means is being recurrently energized.

22. A signal-detection system adapted to prevent a false indication from being given when a characteristically fluctuating event-to-be-detected is not present or when any system component has failed, the system having, in combination, a sensor device adapted to detect the fluctuating condition characteristically produced by the event-to-bedetected and arranged, when influenced by the said condition, to produce a corresponding fluctuating electrical signal, an amplifier connected to the sensor to amplify the said fluctuating electrical signal, electromagnetic means associated with the said amplifier and adapted to be recurrently energized by the amplified fluctuating electrical signal, the electromagnetic means providing maximum response to fluctuating electrical signals of a band of frequencies corresponding to the frequencies of fluctuation of the event, and indicating means responsive to the said electromagnetic means and adapted to give a steady indication during the time that the said electromagnetic means is being recurrently energized, and for a predetermined time greater than the period of the minimum frequency of the said band after the electromagnetic means has ceased to become recurrently energized.

23. A flame-detector system adapted to prevent a false flame signal from being given when a flame is not present o1' when any component of the system has failed, the system having, in combination, a llame-sensor device adapted to detect a fluctuating condition characteristically produced by flames and arranged, when influenced by the said condition, to produce corresponding fluctuating electrical signals, means for applying the said fluctuating electrical signals to an electrical amplifier for amplifying the same, electromagnetic means associated with the said amplifier and adapted to be selectively recurrently energized by amsomos A 13 plied uctuating signals of a band of frequencies corresponding to the frequencies of fluctuation of the llame, and

control means responsive to the said electromagnetic means adapted to give a steady indication during the time 6431370 that the said electromagnetic means is being recurrently 5 2,736,882

energized to indicate that a ame condition is being sensed by the sensor device.

References Cited in the le of this patent UNITED STATES PATENTS Lawrence June 23, 1953 Sorensen Feb. 28, 1956 

